Astronauts and balance-disorder patients can have a common symptom: Disorientation and misperception of motion when moving through the surroundings. Even healthy people can misperceived motion and orientation, especially in an experimental apparatus designed to study self-motion perception. However, little-understood is the way in which the brain interprets the body's linear and angular motions in order to form a perception of motion. Particularly puzzling is the means of distinguishing tilt from translation in the absence of vision. This talk discusses recent research using several experimental apparatus: fixed-radius centrifuge, linear sled, variable-radius centrifuge, and off-vertical axis rotator.